Grandchild process has PID 1 in C - c

Question

I am trying to pass input from parent to child and then from child to grandchild with pipes. After that, the grandchild sends the result to the parent back and the parent will kill both processes.
I can do data passing with pipes but I guess I cannot kill processes because PID of the grandchild is seen as 1 by the parent.
Here is my code:
int main(){
if (pipe(fd) == -1) {
fprintf(stderr,"Pipe failed fd");
return 1;
}
if (pipe(sd) == -1) {
fprintf(stderr,"Pipe failed sd");
return 1;
}
if (pipe(td) == -1) {
fprintf(stderr,"Pipe failed sd");
return 1;
}
printf("Enter a number: ");
scanf("%s", write_msg);
pid_child = fork();
if (pid_child < 0) {
fprintf(stderr, "Fork failed child");
return 1;
}
if(pid_child > 0){ // parent
//pass data to child
close(fd[READ_END]);
write(fd[WRITE_END], write_msg, strlen(write_msg)+1);
close(fd[WRITE_END]);
wait(NULL);
close(td[WRITE_END]);
read(td[READ_END], read_msg_parent, strlen(read_msg_parent)+1);
printf("final output is: %s\n", read_msg_parent);
fflush(stdout);
close(td[READ_END]);
kill(pid_child);
printf("killed child with pid: %d\n" , pid_child);
kill(pid_grandc);
printf("killed grandchild with pid: %d\n" , pid_grandc);
} else { // child + grandchild
pid_grandc = fork();
if (pid_grandc < 0) {
fprintf(stderr, "Fork failed grand child");
return 1;
}
if(pid_grandc > 0){ // child
// get data from parent
close(fd[WRITE_END]);
read(fd[READ_END], read_msg_child, BUFFER_SIZE);
sscanf(read_msg_child, "%d", &x);
printf("Child: input %d and output %d\n", x, x*2);
fflush(stdout);
close(fd[READ_END]);
// pass data to grandchild
close(sd[READ_END]);
x *= 2;
sprintf(write_msg2, "%d", x);
write(sd[WRITE_END], write_msg2, strlen(write_msg2)+1);
close(sd[WRITE_END]);
wait(NULL);
exit(0);
} else { // grandchild
// get data from child
close(sd[WRITE_END]);
read(sd[READ_END], read_msg_grandchild, strlen(read_msg_grandchild)+1);
close(sd[READ_END]);
sscanf(read_msg_grandchild, "%d", &z);
printf("Grandchild: input %d, output: %d\n",z, z*2);
fflush(stdout);
z *= 2;
// pass data to child back
close(td[READ_END]);
sprintf(write_msg3, "%d",z);
write(td[WRITE_END], write_msg3, strlen(write_msg3) + 1);
close(td[WRITE_END]);
exit(0);
}
}
return 0;
}
And the sample output is:
> Enter a number: 12
> Child: input 12 and output 24
> Grandchild: input 24,output: 48
> final output is: 48
> killed child with pid: 7321
> killed grandchild with pid: 1

Answer 1

In your code, since the child is the one that creates grandchild process, the parent doesn't know the grandchild's pid.
I think you can pass the grandchild's pid by using an extra pipe.
after you check:
if(pid_grandc > 0){ // child
passGrandchildPid(newpipename, pid_grandc); // function to write pid to pipe
}
// ...
void passGrandchildPid(int newpipename[2], int pid_grandc){
int pid = grand_child_pid;
close(newpipename[0]); /* Close unused read end */
write(newpipename[1], &pid , sizeof(pid));
close(newpipename[1]);
fflush(stdout);
}
Then in the parent process, you can read the grandchild's process id from this new pipe and kill it.